What do consultants mean when they say "you're not MEESEE?"

MECE (pronounced "meesee") stands for mutually exclusive, collectively exhaustive. Management consultants (McKinsey, BCG, etc.) use it a lot. What does it mean?
In simple language, a text is MECE where are no holes, no overlaps. For example, the following structure is not MECE:

Prices ....

Volume .....

Revenues ....

Revenues is the result of prices x volume, there is an overlap in this list.
Why does being MECE matter? If you want to analyze a problem, it is important that you get all the component structure of the problem right: you need to get them all on the radar screen, and there should not be any dependency between the components. For example if you concluded that "costs are not the issue" and close the analysis here, you don't want it to re-emerge somewhere else in your problem solving effort.
Why does being MECE matter in PowerPoint presentations? If you want to tell the audience a story you don't want to confuse them by bringing up points again in an unclear structure. The logic should be clear.
Should you apply it blindly in your PowerPoint presentations? No. MECE stories can have the perfect logical structure, but can also be boring. For example, in order to be complete, you need to address the rest of the world aftering discussing the most exciting markets US and Asia, even when they are not relevant. Be MECE when designing a story line structure, then adjust to make your presentation interesting and compelling.